Electrophoretic deposition (epd) of radioisotope and phosphor composite layer for hybrid radioisotope batteries and radioluminescent surfaces

ABSTRACT

An electrode for beta-photovoltaic cells includes: a substrate formed of a conductive layer with a thickness ranging between about 10 nm to 1 micron; a composite layer of radioluminescent phosphor with radioisotope particles homogeneously dispersed therein formed on conductive substrate with a thickness ranging between about 1 and 25 microns; and a semiconductor comprising a P-i-N/P-u-N junction or a N-i-P-P junction. The radioisotope may be a beta-emitter, such as Ni-63, H-3, Pm-147, or Sr-90/Y-90.

RELATED APPLICATION DATA

This application is a divisional of U.S. patent application Ser. No.16/366,792 filed Mar. 27, 2019, the disclosure of which is incorporatedby reference its entirety for all purposes.

GOVERNMENT INTEREST

The embodiments herein may be manufactured, used, and/or licensed by orfor the United States Government without the payment of royaltiesthereon.

FIELD

The present invention generally relates a process for formingradioluminescent surfaces. More particularly, embodiments of the presentinvention utilize an electrophoretic deposition process to form acomposite layer of a radioisotope mixed with a radioluminescent phosphoron a substrate. In some embodiments, the substrate may be configured asan electrode for use in radioisotope batteries comprising asemiconductor which generate electrical energy through betavoltaic,beta-photovoltaic and/or photovoltaic processes.

BACKGROUND OF THE INVENTION

Present energy storage is substantially limited by the current chemicalbattery technology. This is due to both extensive infrastructuredevelopment over decades, and wide availability of components andmaterials in the commercial market. Chemical batteries have high powerdensity and can easily power most commercial devices for brief timeperiods. However, chemical batteries suffer from charge leakage,temperature and environment sensitivity, as well as finite charge cyclelimitations.

Radioisotope batteries have the potential to overcome these technicaldeficiencies. They are different from chemical batteries because theyare independent, self-containing energy sources using radioisotopedecay. They produce continuous power over a temperature range, meaningthat they are not limited by diverse environmental conditions.Radioisotope battery energy densities are also several orders ofmagnitudes greater than current chemical batteries. They have thetheoretical ability to remedy deficiencies in current technologies, aswell as introducing new operational capabilities, because of higherenergy density, thermal and mechanical robustness, and a vastly longerlifetime compared to commercially available chemical batteries.

Radioactive decay energy is converted to electrical energy using threemain approaches: direct charge collection and contact potentialdifference, direct energy conversion, and indirect energy conversion.The most efficient of which is the direct energy conversion (DEC) usinga betavoltaic (βV) cell configuration.

In practice, however, the βV cell configuration has suffered from majorsetbacks, including energy conversion efficiency, which is dependent onthe semiconductor material quality and depletion-region volume, betaflux power related radioactivity per area, and effective mass density ofthe radioisotope carrier system.

Thus, indirect energy conversion using a β-PV cell configuration havebeen proposed which utilizes a two-step conversion process convertingnuclear decay to optical energy through phosphor radioluminescence, andthen, optical energy to electrical energy through photoelectric effect.Energy conversion efficiency is determined by the phosphor andphotovoltaic cell efficiency. In practice, however, β-PV cellconfiguration using IDEC have suffered from major setbacks, includingenergy conversion efficiency and geometric constraints.

Thus, there is a need to provide higher power density and improvedconversion efficiency.

SUMMARY OF INVENTION

According to embodiments, a method for forming a phosphor andradioisotope composite layer on a substrate by an electrophoreticdeposition (EPD) process is provided. The method comprises: placing asubstrate formed of conductive material and a counter electrode into acontainer; filling the container with an electrolyte solution having aradioluminescent phosphor particles and radioisotope particles dispersedtherein; connecting the conductive substrate and a counter electrode toa power supply; performing EPD by applying a voltage to the conductivesubstrate and the counter electrode to apply a composite layer ofradioluminescent phosphor with radioisotope particles homogeneouslydispersed therein to the conductive substrate.

In various embodiments, the radioisotope may be a beta-particle emitter,such as Ni-63, H-3, Pm-147, or Sr-90/Y-90. And the radioisotope may bepart of an inorganic or organic compound.

During EPD, the substrate is connected to the negative terminal of thepower supply and the counter electrode is connected to the positiveterminal of the power supply. By using the EPD process, the phosphor andradioisotope particles bond to the substrate without the need for anyadditional binder material. In addition, the method may furthercomprise: coating a surface of the substrate with a photoresistmaterial; applying a pattern defining a cell to the coated surface usinga photolithography process and applying, via the EPD process, the layerof radioluminescent phosphor with radioisotope particles homogeneouslydispersed within the phosphor layer to the substrate to only thepatterned area(s) of the substrate. The method may also include mixingor agitation the electrolyte solution to suspend the phosphor andradioisotope particles therein.

In some exemplary embodiments, (i) the phosphor particles in theelectrolyte solution may range in size from about 100 nm to 20 micronsin diameter; (ii) the radioisotope particles in the electrolyte solutionmay range in size from about 10 nm to 1 micron in diameter; (iii) theconcentrations of the radioluminescent phosphor particles and theradioisotope particles in the EPD solution are about 75 and 25 wt/wt %,respectively; (iv) the thickness of the composite layer formed on thesubstrate by the EPD process may be between about 10 microns to 150microns thick; (v) the packaging density range of the composite layerproduced by the EPD process may be between about 1.8 to 2.1 g/cm³; (vi)the surface uniformity of the composite layer produced by the EPDprocess may be about ±10 microns; and (vii) the composite layer producedby the EPD process may be substantially planar and may provide anoptical power output of approximately 50 nW/cm².

In embodiments where energy is harnessed, the substrate may comprise asemiconductor configured to absorb beta particles and/or photons andoutputting electrical energy. And the electrode may be configured as abetavoltaic or beta-photovoltaic cell having a thicknesses range betweenabout 25 to 100 microns, including the substrate layer thickness.

In embodiments where a radioluminescent surface is produced, theconductive substrate may be comprised of graphene or indium tin oxide(ITO) on quartz, glass, or sapphire with thicknesses between 0.3 to 1 nmand 100 to 200 nm, respectively.

According to further embodiments, an electrode for beta-photovoltaiccells is provided. The electrode may comprise: a substrate formed of aconductive layer with a thickness ranging between about 10 nm to 1micron; a composite layer of radioluminescent phosphor with radioisotopeparticles homogeneously dispersed therein formed on conductive substratewith a thickness ranging between about 1 and 25 microns; and asemiconductor comprising a P-i-N/P-u-N junction or a N-i-P-P junction.

In the case that the semiconductor comprises a P-i-N/P-u-N junction, itmay be comprised of: a GaN or sapphire substrate with a thickness rangefrom 100 nm to 350 μm; a first layer of AlGaN or GaN N-templateapproximately 4 μm in thickness formed on the substrate; a second layerof N—AlGaN approximately 480 nm in thickness formed on the first layer;a third layer of u-AlGaN or i-AlGaN approximately 480 nm in thicknessformed on the second layer; and a fourth layer of P—AlGaN approximately50 nm in thickness formed on the third layer and which the compositelayer is formed on. In some instances, the P-i-N/P-u-N junction dopantconcentrations are 4E17 atoms/cm³ for the further layer of P—AlGaN, 1E16atoms/cm³ for the third layer of u-AlGaN, and 1E18 atoms/cm³ for thesecond layer of N—AlGaN.

In the case that the semiconductor comprises a N-i-P-P semiconductorjunction, it may be comprised of: a GaAs P-Substrate approximately 350μm in thickness; a first layer of P—GaAs approximately 200 nm inthickness formed on the substrate; a second layer of a InGaP bufferlayer approximately 50 nm in thickness formed on the first layer; athird layer of P—InGaP approximately 2000 nm in thickness formed on thesecond layer; a fourth layer of i-InGaP approximately 10 nm in thicknessformed on the third layer; and a fifth layer of N—InGaP approximately 50nm in thickness formed on the fourth layer and on which the compositelayer is formed on. In some instances, the N-i-P-P junction dopantconcentrations may be 1E18 atoms/cm³ for the fifth layer of N—InGaP,1E16 atoms/cm³ for the fourth layer of i-InGaP, and 5E16 atoms/cm³ forthe third layer of p-InGaP.

These and various other embodiments, objects, features, aspects andadvantages of the inventive subject matter will become more apparentfrom the following detailed description of preferred embodiments, alongwith the accompanying drawing figures in which like numerals representlike components.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlyillustrative embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic of the EPD process for forming a phosphor andradioisotope composite layer on a conductive surface of opticallytranslucent or transparent substrate according to embodiments of thepresent invention.

FIG. 2 is a plot of composite layer thickness formed of ⁶³Ni particlesand ZnS:Cu,Al phosphor particles applied on an indium-tin-oxide (ITO)substrate by EPD.

FIG. 3 is an illustration of a composite layer applied by EPD accordingto embodiments of the present invention and the internal interactions.

FIG. 4 shows a composite layer applied by EPD according to embodimentsof the present invention and the net external effect.

FIG. 5 is a plot of photon and beta energy absorption as a function ofZnS:Cu,Al/Ni-63 volumetric configuration composite layer.

FIGS. 6A and 6B are illustrations of hybrid radioisotope battery usingbeta-photovoltaic and betavoltaic energy conversion with volumetricconfiguration.

FIGS. 7A and 7B are illustrations of deposition on planar andthree-dimensional/high aspect ratio semiconductor junction structures.The illustrations are cross sections.

FIG. 8 is a process flow chart of radioisotope battery patterned byphotolithography.

FIG. 9 is a schematic illustration of the EPD process to select whatdevices are beta-photovoltaic and betavoltaic cells.

FIGS. 10A and 10B are illustrations of hybrid radioisotope batterypowering unattended sensor and microprocessor for aerospace, terrestrialapplications, and onboard PCB power.

FIGS. 11A and 11B are illustrations of the composite layer deposited onconductive transparent substrates which are planar and curved,respectively.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate comparable elements that are commonto the figures. The figures are not drawn to scale and may be simplifiedfor clarity. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

The discussion which follows provides a detailed description and exampleembodiments of the inventive subject matter with reference to theaccompanying figures mentioned above.

DETAILED DESCRIPTION

Novel electrophoretic deposition (EPD) processing forms a compositelayer of a radioisotope and a radioluminescent phosphor on a conductivesubstrate. The substrate may be a surface of optically translucent ortransparent substrate. In the composite layer formed, particles of theradioisotope particles are homogeneously dispersed with theradioluminescent phosphor. The composite layer forms one or moreradioluminescent surfaces. In such embodiments, the mixture can bedirectly deposited by EPD on the substrate surface.

The EPD processing may be advantageous for manufacturing radioisotopebatteries with the substrate in which the composite layer is applied andforms on an electrode. In some embodiments, the electrode may becomprised of a semiconductor configured to generate electrical energy.Some of the radioactive particles and/or photons can reach thesemiconductor, the top surface(s) of the substrate should be opticallytranslucent or transparent.

The radioisotope is the energy source and the phosphor and semiconductorare the energy converters. The semiconductor converters can be planarand/or made of high aspect ratio microstructures because of theadvantageous capability of the EPD process. The phosphor andradioisotope mixture configuration provide high radioactivity per volumeand power density through three-dimensional (3D) interaction between theradioactive source, phosphor, and high aspect ratio microstructuresemiconductor transducer. In other applications and embodiments, theradioisotope and phosphor mixture are deposited by EPD on a conductivetransparent surface for use as a light source such as signs and visibleindicators.

Mixing and deposition of radioisotope and phosphor mixture can be lowcost, practical, and provide for ease of manufacture. The mixture can beselectively deposited or applied to any surface and form factor throughlithographic techniques.

The combination of the composite layer of radioisotope andradioluminescent phosphor mixture formed on the semiconductor converterallows it to power electronic solid-state devices, mobile devices,medical devices, unattended sensors, spacecraft instruments,micro-electrical-mechanical systems (MEMS), explosives monitoring, andmicroprocessors for decades.

The EPD process provide greater three-dimensional (3D) interactionbetween the two radioisotope and phosphor particles. In addition,conversion efficiency can be improved by converting radioactive emissionparticles, optical emissions (i.e., photons), or a combination of both,in a suitable semiconductor device structure with the use of a directwide bandgap material.

FIG. 1 is a schematic of the EPD process for forming a phosphor andradioisotope composite layer on a conductive surface of opticallytranslucent or transparent substrate according to embodiments of thepresent invention. This process directly deposits the composite layerdirectly on the substrate without the need for any additional bindermaterial.

The EPD electrolyte 1 is an electrical conductive solution. It is heldin a container 2. In one exemplary embodiment, the EPD electrolyte 1 maybe composed of a solution such as an isopropyl alcohol (IPA) bath with anitrate salt of Mg(NO₃)₂.6H₂O at a concentration of ≈1.6E-4 M. Otherelectrolyte solutions include ethanol, butanol, methanol, and water toname a few. Added salt in solution is dissolved during sonication andstirring in the container 2. The salt should be dissolved before activematerial is added to solution for adequate deposition layers.

The radioluminescent phosphor 3 and radioisotope particles 4 are mixedin the electrolytic solution 1 at certain specified concentrations thatare appropriate for application. A non-limiting example of concentrationis 75/25 wt/wt % of phosphor and radioisotope, respectively.

In various embodiments, the radioluminescent phosphor 3 used in the EPDprocess preferably comprises nano- and micron-sized particles rangingfrom about 100 nm to 20 microns. Examples include infrared-, visible-and ultraviolet-light emitting phosphors include, for instance, zincsulfide doped with copper or silver, strontium aluminate europium anddysprosium doped, and lanthanum phosphate doped with praseodymium.

The radioisotope 4 is selected for a particular application in mind.Radioisotopes decay through three types of particle emission: beta(electron or positron), alpha (atomic nucleus emission), and gamma(electromagnetic radiation).

Beta emitting radioisotopes are the most appealing candidates for energysources, as they do the least amount of damage to the semiconductor(converter) and to the environment. Thus, according to preferredembodiments, the radioisotopes 4 used in the EPD process arebeta-emitter, such as Ni-63, H-3, Pm-147, or Sr-90/Y-90. Theradioisotopes may be in elemental or metallic form, but various saltscompounds (both inorganic and organic) may be also, such as salts,hydrides, and oxides, to name a few. Indeed, as tritium, H-3, isnormally a gas, a tritiated organic compound or a metal compound of H-3such a metal hydride could be used. An example of the tritiated organiccompound is the tritiated nitroxide free radical and trioxylamine. Largemacromolecules, such as polymers, including the radioisotopes may alsobe used. The radioisotopes 4 particles should be solid or dissolvable inelectrolytic solution. Established radioisotope particles and theirproduction are generally known in the art. Different chemical synthesiswill be involved to produce various radioisotope particles. Theradioisotope 4 particles in the electrolyte solution 1 may range in sizefrom about from 10 nm to 1 micron in diameter, for instance.

The EPD solution 1 may be agitated through sonication and/or stirring,for instance, thus suspending particles of the radioluminescent phosphor3 and radioisotope 4. Container 2 should be of sufficient size to holdthe EPD solution 1 and the electrodes 5 and 6. The deposition electrodeor substrate 5 and counter electrode 6 are placed in electrolyticsolution 1. Agitation can be intermittently or continually applied asneeded.

The deposition electrode or substrate 5 is a conductive, opticallytransparent material. It may further be configured as a semiconductordevice for harnessing energy output from the composite layer; suchembodiments are later discussed. The counter electrode 6 should also beconductive; although, its size and shape are not generally important.The composite layer 14 may be formed on all or some portion(s) of thesubstrate 5 generally facing the counter electrode 6. In furtherembodiments, photolithography may be used to selective pattern thesurface of the substrate as discuss below with respect to FIGS. 8 and 9.

During the EPD process, the deposition electrode or substrate 5 isconnected to the negative terminal of the DC power supply 7 and thecounter electrode 6 is connected to the positive terminal of the powersupply 7. The electrode separation distance can be altered for userapplication and deposition rate. Sufficient voltage is applied toeffectuate the EPD process.

The composite layer 14 is deposited directly on deposition electrode orsubstrate 5 using EPD. For example, using ZnS:Cu,Al with averageparticle size diameter of 4 microns and a Ni-63 average particle sizediameter of 500 nm, the average bulk density is about 2.1 g/cm³ appliedusing the EPD process. The composite layer 14 is considered a volumetricconfiguration or light source in that the EPD process creates ahomogenous mixture of nanoparticle radioisotope 8 in a phosphor matrix9. The composite layer 14 thickness may range between about 100 nm to100 microns depending on beta-emitting radioisotope particle size. Whenthe composite layer thickness is sub-micron, the coating may betranslucent or opaque. In addition, surface uniformity is high, meaningthere is less differentiation in layer thickness over substrate surfacearea. For example, the EPD surface uniformity may be approximately ±10microns.

Isotropic beta particle emission 10 creates isotropic photon emission 11in phosphor through radioluminescence. Since the radioisotopenanoparticles are embedded in phosphor layer, a higher amount of photonsare created in the volumetric due to greater interaction betweenradioisotope and phosphor. Volume radioactivity and phosphor thickness11 can be greater than thin film phosphor thickness 12 because ofgreater photon range 11 in volume compared to beta range. There is lessphoton attenuation in the phosphor, thus more photons can exit thevolume compared to a thinner phosphor layer 12. Volumetric configuration13 has a greater energy and power density than the two-dimensionalthin-film configuration because more photons are produced in the largervolume. Volumetric configuration 13 overcomes geometric constraints fromthin film configuration by spreading radioisotope throughout phosphormatrix. The optical power can be increased with the volumetricconfiguration using the EPD process. For example, using Ni-63 withZnS:Cu,Al produces approximately 50 nW/cm².

In general, the deposition rate can be increased through an increase inDC voltage (VDC) across the electrodes and/or reducing the separationdistance of the electrodes.

FIG. 2 is a plot of composite layer thickness formed of ⁶³Ni particlesand ZnS:Cu,Al phosphor particles applied on an indium-tin-oxide (ITO)substrate by EPD. The plot show two electrical potential sets for 40 and100 VDC. Higher electrical potential between electrodes increasesdeposition rate. Both sets appear to show a saturation trend at thenearly same deposition time.

The deposition electrode or substrate 5 which the composite layer 14 wasapplied and counter electrode 6 were spaced apart with a separationdistance of 8 millimeters. This phenomenon will change with alterationof electrode, phosphor, radioisotope, and electrolytic solution type,electrode separation distance, electrolytic solution agitation process,and voltage.

FIG. 3 is an illustration of a composite layer applied by EPD accordingto embodiments of the present invention and the internal interactions.It is comprised of beta-radioisotope nanoparticles 8 andradioluminescent phosphor. Since the phosphor is in much larger quantitythan the beta-radioisotopes, it form a matrix 9. The radioisotopenanoparticles 8, through radioactive decay, emits beta particleemissions 10. The beta particle emissions 10 react with theradioluminescent phosphor to emit photon emissions 11. The spectralcharacterization of the photon emissions 11 will depend of theparticular radioluminescent phosphor selected. Both beta and photonemissions are isotropic.

The volumetric configuration of the beta-radioisotope nanoparticles inradioluminescent phosphor matrix provides a great deal of interactionbetween phosphor and radioisotope. The photon range is greater than betarange in material because less photon attenuation. Photons are able totravel deeper in phosphor matrix with minima attenuation of about 50 to100 microns. Beta particles from 1 to 16 keV are only able to penetrateabout 1 to 20 microns in phosphor layer depending on mass and bulkdensity.

FIG. 4 shows a composite layer applied by EPD according to embodimentsof the present invention and the net external effect. Many photons arecreated in volumetric configuration due to three-dimensional interactionbetween phosphor and radioisotope nanoparticles 8 which are output. Thethickness 13 of the composite layer is selected to emit substantiallyonly photons 11.

FIG. 5 is a plot of optical (photon) and beta energy absorption as afunction of distance (or thickness) of the composite layer formed byEPD. For the data presented, ZnS:Cu,Al/Ni-63 volumetric composite layerwas formed on an electrode using the EPD process. The Ni-63 was in theform of the salt, nickel-63 chloride (⁶³NiCl₂). The Ni-63 emits betaparticles. The three-dimensional interaction of beta particles from theNi-63 creates photons from the phosphor particles through ionizationprocesses (radioluminescence).

The measured energy penetration depth in ZnS:Cu,Al for beta particleemission from Ni-63 and photon (525 nm wavelength) shows that photonshave a greater range before being absorbed in the phosphor. Emissions byboth mechanisms occur between a range of approximately 1 to 25 micronslayer thickness, which is considered the overlapping region.

The beta particle penetration depth for 90% of the beta energy to beabsorbed in the ZnS:Cu,Al at bulk density of 2.1 g/cm³ is approximately10 microns with nickel-63 chloride (⁶³NiCl₂). This would normally limitthe ZnS:Cu,Al layer thickness to approximately 10 microns. However, bymixing the radioisotope with the phosphor, the beta particles producephotons in the layer that can travel approximately 400 to 500 micronsbefore being 90% self-absorbed. The increase of layer thickness by 40×enables an increase in the energy deposited and the energy converted fora higher electrical power output. A unique feature of the EPD process isthat the composite layer thickness can be modified to maximize bothoptical (photon) and beta penetration depths, and thereby maximizeelectrical power out of the device.

By judiciously configuring the electrode 5 and the composite layer 14together, the energy output from the composite layer 14 can beefficiently harnessed by the electrode 5 in a radioisotope battery. Thethickness of composite layer 14 determines if the device is abetavoltaic (β-V), beta-photovoltaic (β-PV) or photovoltaic (PV) cellradioisotope battery based on its output. For the data shown in FIG. 5,a thinner composite layer (e.g., 0-1 μm) emits primarily beta particles15, whereas a thicker composite layer (e.g., above ˜25 μm) emitsprimarily only photons 11, 16. In between (˜1-25 μm), the compositelayer 14 emits some combination of both beta particles 15 and photons11, 16.

Direct bandgap semiconductors convert beta particles and/or photons intousable electrical energy. Depending on the radioisotope batteryconfiguration, betavoltaic, beta-photovoltaic and/or photovoltaicprocesses occur. The radioisotope battery may be composed of multiplecells electrically connected in series or parallel.

The radioactive isotope particles emit beta particles, which are similarto electrons. Suitable energy conversion processes may be employed by asuitable semiconductor depending of the external emission of thecomposite layer 14. As previously stated, based on its thickness, thecomposite layer 14 may emit photon, beta particles, or some combinationthereof.

A betavoltaic process may be used to convert beta particles toelectrical energy, a beta-photovoltaic process to covert beta particlesand photons to electrical energy, and a photovoltaic processes to covertphotons to electrical energy. Energy conversion efficiency is determinedby the phosphor and photovoltaic cell efficiency. For example, ageometric constraint is the limited interaction between the converterand radioactive material. The most efficient of these is the directenergy conversion using a betavoltaic cell (βV) configuration. But,there may be other reasons to use the other approaches.

Phosphor emitted photons can be absorbed by a photovoltaic cell, such aPN semiconductor junction. Indirect energy conversion is determined bythe interaction between the radioisotope material and the twoconverters: phosphor and photovoltaic cell. The phosphor layer is anenergy medium between beta particles and PN junction limiting themagnitude of radiation damage and converting the entire beta spectruminto usable photons, especially, when using higher beta energyradioisotope sources such as Sr-90/Y-90 and Pm-147.

Beta emissions may be harnessed by a suitable semiconductor materialsthrough the carrier system, such as a Schottky diode, PN junctionsemiconductor, P-i-N/P-u-N junction semiconductor, or N-i-P-P junctionsemiconductor. Similar to photovoltaic cells, electron-hole pairs(e-h-ps or ehps) are created by the ionization trails of the incidentbeta particles inside or within a minority carrier diffusion length ofthe depletion region. They are separated by the built-in electric fieldfrom P-type junction and N-type junction; electron hole pairs (ehps) aredrifted are apart. The gathering of separated ehps in the neutral regionof the semiconductor where the electrons are on the N-junction side andthe holes are on the P-junction side results in the junction becomingforward biased and current flowing through an externally connected load.

Beta-photovoltaic (β-PV) conversion uses a two-step conversion processto convert (i) nuclear decay to optical energy through phosphorradioluminescence, and then, (ii) optical energy to electrical energythrough photoelectric effect in a PN-junction photovoltaic cell. As usedherein, the term “hybrid” may be used to refer to semiconductor capableof converting both betas and photons into usable electrical energyfollowing typical processes of both photovoltaic (PV) and betavoltaic(βV) devices.

Variation of the thickness of the radioisotope and phosphor compositelayer 14 allows an electrode for a hybrid radioisotope battery to bedeveloped, taking advantage of both DEC and DEC. Radioisotopes, such asNi-63 and Pm-147, are solid at STP and emit higher energy beta particles(18-250 keV) that are more suitable for β-PV and βV configuration andlight sources.

The EPD process described herein, when used with phosphors mixed withbeta-emitting radioisotopes, enables a hybrid radioisotope battery to becreated in some embodiments and optimized for power output and materialcosts. For example, the EPD process enables varying thicknessoptimization of the phosphor and radioisotope mixture/composite to becreated on the surface of the electrode substrate, such as InGaPsemiconductor.

FIGS. 6A and 6B show two hybrid radioisotope battery embodiments whichuse beta-photovoltaic and betavoltaic energy conversion with avolumetric configuration according to embodiment. In these drawings, thenomenclature i/I represents the intrinsic layer and u/U represents theundoped layer. In the semiconductor field, they are used to representthe same layer or junction. Thus, P-i-N or P-u-N refer to the same typeof junction.

The hybrid radioisotope battery in FIG. 6A includes a P-i-N or P-u-Njunction semiconductor 17, such as mc-Si, a-Si, GaAs, InGaP, GaP, GaN orAlGaN (direct wide bandgap semiconductors). It is configured for acomposite layer which emits a combination of beta particles and photons.The P-i-N/P-u-N junction is comprised of: a GaN or sapphire substratewith a thickness range from 100 nm to 350 μm; a first layer of AlGaN orGaN N-template approximately 4 μm in thickness formed on the substrate;a second layer of N—AlGaN approximately 480 nm in thickness formed onthe first layer; a third layer of u-AlGaN or i-AlGaN approximately 480nm in thickness formed on the second layer; and a fourth layer ofP—AlGaN approximately 50 nm in thickness formed on the third layer andwhich the composite layer is formed on. For example, the dopant levelsof AlGaN device are 4E17 atoms/cm³ for p-AlGaN, 1E16 atoms/cm³ foru-AlGaN, and 1E18 atoms/cm³ for n-AlGaN.

The hybrid radioisotope battery in FIG. 6B is configured for a thickercomposite layer which emits just photons. It includes a N-i-P-P junctionsemiconductor 18, such as GaP, InGaP, GaAs, a-Si, and mc-Si. The N-i-P-Pjunction 18 is comprised of a GaAs P-Substrate approximately 350 μm inthickness; a first layer of P—GaAs approximately 200 nm in thicknessformed on the substrate; a second layer of a InGaP buffer layerapproximately 50 nm in thickness formed on the first layer; a thirdlayer of P—InGaP approximately 2000 nm in thickness formed on the secondlayer; a fourth layer of i-InGaP approximately 10 nm in thickness formedon the third layer; and a fifth layer of N—InGaP approximately 50 nm inthickness formed on the fourth layer and on which the composite layer isformed on. For example, the dopant levels of InGaP device are 1E18atoms/cm³ for n-InGaP, 1E16 atoms/cm³ for i-InGaP, and 5E16 atoms/cm³for p-InGaP.

In both embodiments, electron hole pairs are created and collectedcausing an electric potential difference. Non-conductive surfaces willeither be partially or not covered by composite coating using EPDprocess.

The EPD process can deposit radioisotope and phosphor composite layer onspecific individual devices of wafer. FIGS. 7A and 7B show two suchapplications. Composite layers 13, 14 can be directly deposited onvarious conductive surfaces with different aspect ratios such as planar19 structure (FIG. 7A) or a pillared 20 structure (FIG. 7B). Ahoneycombed structure could similarly be produced.

In some embodiments, a photolithography process is used inmicrofabrication to pattern certain area of the substrate with thinlight-sensitive chemical “photoresist” material. Photolithography isgenerally known in the art.

FIG. 8 shows a process of how the patterning a deposition electrode orsubstrate by photolithography. First, a substrate wafer is prepared andcleaned, for instance, by a wet chemical treatment. Photoresist isapplied on wafer surface, such as, through spin coating. The spincoating typically runs at 1200 to 4800 rpms for 10 to 30 secondsproducing a layer between 100 nm to 1 micron. The spin coating processresults in a thin layer with uniformity of within 5 to 10 nm. The photoresist-coated wafer is then prebaked to get rid of excess photoresistsolvent laying on the top, typically at 90 to 100° C. for 30 seconds onhotplate. The photo resist-coated wafer is then exposed to a pattern ofintense light and removed using a special solution called a “developer”.

As shown in FIG. 9, exposed areas 21 are the conductive areas of thewafer, whereas areas 22 covered with the photoresist are non-conductive.The photo resist-coated wafer, used as the deposition electrode orsubstrate 5, along with a counter electrode are placed in electrolyticsolution for EPD process. Composite layer is deposited on exposed areas23.

In addition, composite layer can be deposited on a single device throughelectrical connection of P-layer, which is a unique junction for eachdevice. Typically, semiconductor devices have common N-layer orjunction. If N-junction is electrically connected during EPD process,all exposed areas are coated with composite layer.

The hybrid radioisotope battery, using volumetric configuration andseveral connected semiconductor devices, could power solid stateelectronics. Example of these device are shown in FIGS. 10A and 10B.Depending on beta-emitting radioisotope and semiconductor materialquantity and quality, battery 24 can power mobile devices, medicaldevices, unattended sensors 25, spacecraft instruments, MEMS, explosivesmonitoring, and microprocessors. The battery can be directly placed onthe circuit as onboard power for sensor sleep mode or active mode. Thebattery is electronically and mechanically connected to the circuitboard. The battery would resemble a basic dual in-line package orsurface mount IC chip 26. The batteries have the potential to bemaintenance free power sources for remote, long term, low power sensors.Because current sensors' microprocessors 26 have low quiescent powerrequirement for sleep mode ranging from 1 to 10 microwatt of electricalpower, a small quantity of material (beta source and semiconductor) isable to continuously power sensor throughout its entire lifetime. Thepower source will have a lifetime of the sensor network orinfrastructure rather than itself. This could introduce widespreadintelligent sensor networks that could monitor enormous amounts area ofdiverse environments and have the constant power to rely valuableinformation back using communicate nodes.

For luminescent applications, the composite layer 27 can be deposited onany conductive substrate that is transparent in the IR, UV and/orvisible light spectrum as an application may need. This is shown inFIGS. 11A and 11B. Examples of these conductive substrates are grapheneon quartz or glass and indium tin oxide (ITO) on quartz and glass. ITOand graphene layer 28 are several nanometers thick on glass or quartz29, which is typically less than a millimeter thick. The substrate canbe flat or curved.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the embodiments herein that others can, byapplying current knowledge, readily modify and/or adapt for variousapplications such specific embodiments without departing from thegeneric concept, and, therefore, such adaptations and modificationsshould and are intended to be comprehended within the meaning and rangeof equivalents of the disclosed embodiments. It is to be understood thatthe phraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Therefore, while the embodimentsherein have been described in terms of preferred embodiments, thoseskilled in the art will recognize that the embodiments herein can bepracticed with modification within the spirit and scope of the claims.

We claim:
 1. An electrode for beta-photovoltaic cells comprising: asubstrate formed of a conductive layer with a thickness ranging betweenabout 10 nm to 1 micron; a composite layer of radioluminescent phosphorwith radioisotope particles homogeneously dispersed therein formed onconductive substrate with a thickness ranging between about 1 and 25microns; and a semiconductor comprising a P-i-N/P-u-N junction or aN-i-P-P junction.
 2. The electrode of claim 1, wherein the P-i-N/P-u-Nsemiconductor junction comprises: a GaN on sapphire substrate with athickness range from 100 nm to 350 μm; a first layer of AlGaN or GaNN-template approximately 4 μm in thickness formed on the substrate; asecond layer of N—AlGaN approximately 480 nm in thickness formed on thefirst layer; a third layer of u-AlGaN or i-AlGaN approximately 480 nm inthickness formed on the second layer; and a fourth layer of P—AlGaNapproximately 50 nm in thickness formed on the third layer and which thecomposite layer is formed on.
 3. The electrode of claim 1, wherein theN-i-P-P semiconductor junction comprising: a GaAs P-Substrateapproximately 350 μm in thickness; a first layer of P—GaAs approximately200 nm in thickness formed on the substrate; a second layer of a InGaPbuffer layer approximately 50 nm in thickness formed on the first layer;a third layer of P—InGaP approximately 2000 nm in thickness formed onthe second layer; a fourth layer of i-InGaP approximately 10 nm inthickness formed on the third layer; and a fifth layer of N—InGaPapproximately 50 nm in thickness formed on the fourth layer and on whichthe composite layer is formed on.
 4. The electrode of claim 2, whereinthe P-i-N/P-u-N junction dopant concentrations are 4E17 atoms/cm³ forthe further layer of P—AlGaN, 1E16 atoms/cm³ for the third layer ofu-AlGaN, and 1E18 atoms/cm³ for the second layer of N—AlGaN.
 5. Theelectrode of claim 3, wherein the N-i-P-P junction dopant concentrationsare 1E18 atoms/cm³ for the fifth layer of N—InGaP, 1E16 atoms/cm³ forthe fourth layer of i-InGaP, and 5E16 atoms/cm³ for the third layer ofp-InGaP.
 6. The electrode of claim 1, wherein the radioisotope is abeta-particle emitter.
 7. The electrode of claim 6, wherein thebeta-emitter radioisotope comprises Ni-63, H-3, Pm-147, or Sr-90/Y-90.8. The electrode of claim 1, wherein the radioisotope is part of aninorganic or organic compound.
 9. The electrode of claim 1, where theradioluminescent phosphor particles and separate and distinct from theradioisotope particles.
 10. The electrode of claim 1, wherein theradioluminescent phosphor particles range in size from about 100 nm to20 microns in diameter.
 11. The electrode of claim 1, wherein theradioisotope particles range in size from about from 10 nm to 1 micronin diameter.
 12. The electrode of claim 1, wherein the concentrations ofthe radioluminescent phosphor particles and the radioisotope particlesare about 75 and 25 wt/wt % based on solid contents, respectively. 13.The electrode of claim 1, wherein the packaging density range of thecomposite layer is between about 1.8 to 2.1 g/cm³.
 14. The electrode ofclaim 1, wherein the surface uniformity of the composite layer is about±10 microns.
 15. The electrode of claim 1, wherein the composite layeris substantially planar and provides an optical power output ofapproximately 50 nW/cm².
 16. The electrode of claim 1, wherein thesubstrate comprises a semiconductor configured to absorb beta particlesand/or photons and output electrical energy.
 17. The electrode of claim1, wherein the substrate comprises graphene, indium tin oxide (ITO) onquartz, glass, or sapphire.